The art is replete with techniques for using sensors to monitor the performance of a wide variety of industrial and other processes and for calibrating and checking on the zero and span characteristics of the sensors or comparing the same with reference impedances and the like. Among such are, for example, single transmission line-interrogated multiple channel data acquisition systems, as disclosed in U.S. Pat. No. 4,196,417; systems for alternately connecting one or more reference impedances and sensor impedances to a measuring circuit as in U.S. Pat. No. 4,751,654; systems using multiplexing with sample-and-hold test sensors as in U.S. Pat. No. 4,005,273; and systems employing multiplexed signal reference level equalizers, output correction circuits for analog sensors, and/or other analog sensing and signal processing circuitry in current loops, as respectively described in U.S. Pat. Nos. 4,056,686; 4,930,095; and 4,783,659, and the like.
While providing various degrees of sensor monitoring, checking or calibration, however, the prior art appears not to have provided a technique for automatically diagnosing the operational correctness and viability of sensors as they are, in turn, being used to monitor and determine the viability of processes and the like, such as, for example, the temperature, pressure or other parameters involved in the various parts or steps of a manufacturing or other critical process of concern.
The present invention addresses this deficiency by providing for automated sensor diagnosis through quantitative measurement of one of sensor-to-earth conductance (or reciprocal of impedance, more generally), or loop impedance (resistance) of the sensors before and during the sensor monitoring or validating of the process. The term "earth" is also generically used herein to connote grounding, chassis or other reference potential.